(i) Technical Field
The present invention relates to diaphragm devices.
(ii) Related Art
Japanese Unexamined Patent Application Publication No. 2005-275177 discloses a diaphragm device in which plural blades reduce an aperture diameter of an opening formed in a board. A drive force is transmitted from a step motor to the blades through a drive ring. The drive force is transmitted from the step motor to the drive ring through a teeth portion formed in the drive ring or a gear. The drive force is transmitted from the drive ring to the blades by engaging drive pins of the drive ring with cam slots formed in the blades.
A rotor of the step motor used for the diaphragm device is capable of stopping at given step angle intervals. The plural positions where the rotor is capable of stopping are set. The rotor stops, and then the drive ring and the blades stop. The aperture diameter of the opening is defined based on the stop positions of the blades.
Generally, the drive force of the rotor is transmitted to the drive ring through a gear or the like. Thus, there is a linear relationship between the rotational angle of the rotor and the rotational angle of the drive ring. Therefore, since the intervals between the adjacent positions where the rotor is capable of stopping are constant, the intervals between the adjacent positions where the drive ring is capable of stopping are also constant.
On the other hand, an interval between the adjacent stop positions of the blade in response to the stop of the rotor is set to be smaller as the aperture diameter of the opening is reduced. This is achieved, because the drive ring moves such that its drive pin is more spaced apart from a rotational center of the blade as the aperture diameter of the opening is reduced. This is because the rotational angle of the blade about the rotational center becomes smaller than a given rotational angle of the drive pin, as the distance between the rotational center of the blade and the drive pin is larger.
When the drive ring stops at a position misaligned from the original stoppable position, the blade also stops at a position misaligned from the original stop position. This influences the aperture diameter of the opening. In particular, the stop position is misaligned in such a way in the minimum aperture state where the aperture diameter of the opening is minimum, so that the accuracy of the aperture diameter of the opening is significantly influenced. In order to make the influence on the accuracy of the aperture diameter of the opening as less as possible, it is preferable that the intervals between the stoppable positions of the drive ring should be set as large as possible and the distance between the rotational center of the blade and the drive pin of the drive ring should be set as large as possible in the minimum aperture state. With the above setting, the interval between the adjacent stoppable positions of the blade is minimum in the minimum aperture state, relative to the interval between the adjacent stoppable positions of the drive ring. Thus, even when the drive ring is misaligned from the stop position in the minimum aperture state, the stop position of the blade is less influenced.
Herein, the intervals between the stop positions of the drive ring are constant in the whole movable range of the drive ring. This is because, as mentioned above, the step motor stops at constant intervals, and the drive force is transmitted to the drive ring through a gear from the step motor in a general manner. Thus, if the intervals between the stoppable positions of the drive ring are set as large as possible, the whole movable range of the drive ring is made large, so the whole movable range of the drive pin is made large. In accordance with this, the size of the cam slot engaging the drive pin has to be increased. This increases the size of the blade.
As mentioned above, if the accuracy of the stop position of the blade is maintained in the minimum aperture state where the aperture diameter of the opening is minimum, the movable range of the drive ring is increased, so the size of the blade is increased.